1. Field of the Invention
The present invention relates to riser management systems. More particularly, the present invention relates to systems, computer readable media, program code, and related methods for monitoring and managing a plurality of marine riser assets.
2. Description of Related Art
A problem presented by offshore hydrocarbon drilling and producing operations conducted from a floating platform or vessel is the need to establish a sealed fluid pathway between each borehole or well at the ocean floor and the work deck of the vessel at the ocean surface. This sealed fluid pathway is typically provided by a drilling riser system. Drilling risers, which are utilized for offshore drilling, extend from the drilling rig to a blowout preventer (BOP). Similarly, production risers extend from and provide communication between a subsea wellhead system and the floating vessel.
A typical marine drilling riser permits passage of drill pipe which is used for pumping lubricating mud down the well during drilling operations, return of drilling mud that has been pumped through the drill pipe into the main tube of the riser, and any associated drill cuttings, and provides a connection of the drilling vessel to the well above the subsea BOP stack. The drilling riser can be disconnected from the well above the BOP stack, allowing the drilling vessel to retrieve the riser and temporarily move from the drill site should the need arise (i.e., during a hurricane event, or a malfunction). The BOP stack, having remained on the wellhead, provides for containment of a live well while the vessel is not on location. Upon return, the vessel can deploy the riser, reconnect to the BOP stack, and reestablish hydrocarbon communication with the well.
The marine drilling riser also permits control of the well should the BOP stack have to be functioned. This is typically associated with drilling through a zone with geological fluid pressure that is substantially higher than that which the drilling mud can contain. During such events, the BOP is functioned, and well control is re-established by pumping an appropriate density mud thought the kill line and eventually circulating it back to the surface via the choke line. The marine drilling riser also permits improvement of mud circulation velocity. When needed, this is accomplished by pumping additional mud through the booster line and injecting it into the riser bore at the BOP stack. This increases the volume of mud in the riser; improving the return speed. The marine drilling riser further permits delivery of hydraulic fluid to the BOP stack control system. Such fluid is supplied through a dedicated external hydraulic line.
The drilling riser, for example, is typically installed directly from a drilling derrick on the platform of the vessel by connecting a series of riser joints connected together. After connecting the riser to the subsea wellhead on the seabed, the riser is tensioned by buoyancy cans or deck mounted tensioner systems. The riser is projected up through an opening, referred to as a moon pool in the vessel, to working equipment and connections proximate an operational floor on the vessel. In drilling operations, the drill string extends through a drilling riser, with the drilling riser serving to protect the drill string and to provide a return pathway outside the drill string for drilling fluids. Similarly, in producing operations, a production riser is used to provide a pathway for the transmission of oil and gas to the work deck.
Basic components of a riser system typically include, from the mud line and extending to the surface: a hydraulic wellhead connector which permits connection to a subsea wellhead; a BOP stack used for well control; a lower marine riser package which permits disconnect and reconnect of the marine riser at the BOP stack; multiple marine riser joints normally in the form of bare and buoyant joints each outfitted with a choke and kill line, a booster line, and a hydraulic line; and a termination joint which is a special riser joint where external lines are terminated and diverted to the appropriate facility on the vessel. For example, the kill line is terminated and connected to the mud pump via a high pressure flexible line. The components also include a tension ring which provides for the interface of the marine riser to the hydro-pneumatic riser tensioner designed to provide lateral load resistance while providing a somewhat constant vertical tension; and a telescopic joint typically made of two sliding pipes sealed together via an elastomer primarily used is to decouple the motion of the vessel, while permitting the riser tensioner system to apply a near constant tension on the marine riser. The components also include a diverter used for diverting of the unwanted gas in the marine riser; a gimbal located on the rig floor used during running and retrieval of the marine riser to dampen the pitch motion of the vessel, and a marine drilling riser spider used during the mating of each riser section to the next.
Other more specialized riser equipment includes a fill-up valve designed to prevent collapse of the riser pipe due to the differential pressure between the inside of the riser pipe and the surrounding water, an instrument riser joint typically used to monitor the tension and bending due to environmental conditions which allows for adjustment in top tension and vessel positioning, vortex suppression equipment which help suppress vortex induced vibrations typically found in conditions of high current and long riser length, and an emergency riser release which provides a specialized riser release system to prevent catastrophic failure typically found in conditions where incorrect vessel positioning or extreme environmental conditions may occur.
During a typical field installation, the marine riser components are individually lifted from the deck, connected to each other at the riser spider, and run down. Riser joints, which comprise the major length of the riser string, are fabricated in lengths ranging from 50′ to 90′. During the running procedure, the portion of the riser string that is fully made up is landed on the riser spider. The next riser joint is then picked up and placed just over the spider, immediately above the suspended riser string. The two riser sections are then joined by means of a mechanical connector, etc. The most common type for a riser joint provides a bolted flange configuration.
Marine risers are subjected to impact loads as well as unexpected side loads, which can damage fragile electronics. Marine risers are also subjected to environmental loads as well as vessel-induced loads. The associated environmental parameters include, among other things, wave height and period, water depth, current, wind, and tides. In the subsea environment, hydrostatic pressure can reach 4,500 psi in current deepwater areas, and probably will reach 5,500 psi within a few years, and the seawater temperature can be as cold as 30° F. The high temperature of the drilling mud could also impact electronic and sensor equipment, particularly electronic equipment attached directly to the riser pipe (joint) body. The vessel-induced loads include the applied top tension necessary to maintain the optimum shape for a riser string, and those imparted by the marine riser string due to motion of the vessel as it is subjected to wave, wind, and current loading. The most critical component of environmental loads is generally the current load directly imparted on the riser string. The current loads typically vary with the water depth, but are generally much stronger near the surface.
Some locations around the world, such as the West of the Shetlands and the Gulf of Mexico, offer unique challenges associated with strong currents. In the Gulf of Mexico, for example, an environmental event establishes a seasonal “Loop Current”, which moves in a circular pattern reaching a diameter a hundred miles or more. Such loop currents impart exceptionally high environmental loads on a riser, often for weeks at a time. High current loading can result in shedding of vortices past a marine riser string, which, if coupled with unfavorable riser damping, can result in violent motion of a marine riser string, typically in a cross flow direction. This is commonly referred to as “Vortex Induced Vibration” or “VIV.” The large amplitude of the riser motion during a VIV event can result in elevated stress levels in the riser string, which, in turn, dramatically reduces the fatigue life of the individual riser joints. A single fatigue event can potentially result in the catastrophic failure of the marine riser string. Worse yet, a VIV event can take place in higher modes, for example, such as where only a small portion of the riser string, possibly hundreds of feet below the water surface, is excited and experiences VIV. Recognized by the inventor is that in such a scenario, an observer on the vessel, looking down at the visible portions of the riser string, would see no evidence of this VIV event. Fortunately, catastrophic failure of risers has been few and far between.
A goal or series of goals for both drilling and production risers is to manage stresses and loading of individual riser sections to provide for fatigue analysis, and thus, allow the operator to formulate an enhanced inspection, maintenance, and riser joint rotation program. If a single riser joint in a riser string fails or otherwise exceeds an operational constraint resulting in a requirement for immediate maintenance, an entire riser string may have to be retrieved and rerun. In deepwater operations, it might take two or more days to run or retrieve a marine riser. Given the approximate rate of well over $500,000 per day for a 5th generation drilling vessel, such a scenario would cost the operator over a million dollars just to establish communication with the subsea well. This point should illustrate the importance of saving time during the running and retrieving of the riser. It should also illustrate the importance of lost time, and the associated cost, resulting from a riser component failure. There have been, however, until now, no effective systems or methods of efficiently tracking riser assets, efficiently tracking cumulative stress or other loading on each riser asset, or accurately determining or differentiating expected stress levels between vessels or fields in order to properly forecast required maintenance.